Dual polarization waveguide probe system

ABSTRACT

A waveguide includes a waveguide body, a twist plate, and a first and second probes. The waveguide body defines a waveguide cavity therein wherein the waveguide cavity has an aperture at a first end thereof, and wherein the waveguide cavity has a waveguide axis therethrough extending from the first end to a second end. The twist plate is in the waveguide cavity at the second end of the waveguide cavity wherein the twist plate is parallel to the waveguide axis, wherein the twist plate includes a leading edge facing the aperture, and wherein the leading edge includes first and second portions with the second portion being more distant from the aperture than the first portion. The first probe is in the waveguide cavity between the aperture and the leading edge of the twist plate for receiving a first signal having a first polarization entering the aperture. The second probe is in the waveguide cavity between the first probe and the leading edge of the twist plate for receiving a second signal having a second polarization entering the aperture. Related receivers and methods are also discussed.

FIELD OF THE INVENTION

The present invention relates to a dual polarisation waveguide probesystem for use with a satellite dish for receiving signals broadcast bya satellite which include two signals orthogonally polarised in the samefrequency band. In particular, the invention relates to an improvedwaveguide for use with a low-noise block receiver into which two probesare disposed for coupling from the waveguide desired broadcast signalsto external circuitry.

BACKGROUND OF THE INVENTION

In applicant's co-pending Published International Application WO92/22938there is disclosed a dual polarisation waveguide probe system in which awaveguide is incorporated into a low-noise block receiver in which twoprobes are located for receiving linearly polarised energy of bothorthogonal senses. The probes are located in the same longitudinal planeon opposite sides of a single cylindrical bar reflector which reflectsone sense of polarisation and passes the orthogonal signal with minimalinsertion loss and then reflects the rotated orthogonal signal. Theprobes are spaced λ/4 from the reflector. A reflection rotator is alsoformed at one end of the waveguide using a thin plate which is orientedat 45° to the incident linear polarisation with a short circuit spacedapproximately a quarter of a wavelength (λ/4) behind the leading edge ofthe plate. This plate splits the incident energy into two equalcomponents in orthogonal planes, one component being reflected by theleading edge and the other component being reflected by the waveguideshort circuit. The resultant 180° phase shift between the reflectedcomponents causes a 90° rotation in the plane of linear polarisationupon recombination so that the waveguide output signals are located inthe same longitudinal plane.

The above waveguide probe system has been found to perform well for thepurpose for which it was designed; to provide significant signalisolation better than 40 dBs. across the current Astra satellitebandwidth being 10.7-11.8 GHz. and across other bandwidths such as11.7-12.2 GHz. for DBS and 12.2-12.75 GHz. However, there has been atrend to increase the frequency range transmitted by new satellitesystems. In fact, the frequency bandwidth is planned to increase from10.7-11.8 GHz. to 10.7-12.75 GHz. on the Astra system in the nearfuture. With the aforementioned design it has hitherto been difficult touse a single LNB or waveguide to cover this wider frequency range, thefrequency range being covered by two or more LNBs which are tuned tocover part of the frequency range, for example 10.7-11.8 GHz. and11.7-12.2 GHz. The existing LNB may be frequency limited because of thebandwidth achieved by the reflection rotation of the existing design.

JP-A-02029001 discloses a waveguide system which is used to rotate andreflect a signal. One embodiment of this system uses a steppeddielectric plate, which is non-reflecting, to introduce a phase shift of180° for one component of the signal relative to the orthogonalcomponent. This reference discloses an alternative embodiment which usesa capacitive metal rod or a dielectric rod on the diagonal line of thewaveguide cross-section instead of a stepped dielectric plate. Theparticular solution to this problem may require a dielectric plate orrod or a capacitive metal rod.

GB 2 076 229 discloses the use of a stepped plate in apparatus forconverting circularly polarised signals in a square waveguide intolinearly polarised signals. It is a modified form of septum polariserwhich is well known in the art, and may not relate to reflection andrecombination of signals to provide an increased frequency range ofoperation.

FR 2 615 038 discloses a waveguide with a vane which acts as a shortcircuit to one of the coaxial probes. The apparatus may not providephase rotation and recombination and may not be suitable for providingan increased frequency range of operation.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved dualpolarisation waveguide probe system which reduces at least one of theaforementioned disadvantages.

It is a further object of the invention to provide a dual polarisationwaveguide probe system which can cover all Astra satellite bandwidths ina single LNB.

It is a further object of the present invention to provide an improveddual polarisation waveguide probe system with equivalent ease ofmanufacture to the existing waveguide probe system.

This can be achieved by providing a reflective twist plate within theprobe housing which has at least two signal reflecting edges so that atleast two separate signal reflections are created. The multiple signalreflections can enable the probe system to operate over a widerfrequency range with minimal deterioration in signal output.

In a preferred arrangement, this is achieved by making the reflectingtwist plate stepped and by providing two steps spaced at differentdistances from the waveguide short circuit. The leading, reflecting,edges of the steps are orthogonal to the waveguide axis. In analternative arrangement, the reflecting twist plate may be replaced by athree step reflecting edge or by a castellated edge such that there aremultiple spaced reflecting edges. This can be achieved by casting aprobe system in which the waveguide has a two or three step reflectingtwist plate. Alternatively, the single reflecting edge of an existingtwist plate may be drilled to a predetermined depth into the twist plateto create separate reflecting edges.

Alternatively, the reflecting edge may be provided by a continuousleading edge such as an oblique line or a curve or a series of curves.

According to a first aspect of the present invention there is provided awaveguide into which at least two orthogonally polarised signals arereceived for transmission therealong, said waveguide having;

a first probe extending from a wall of the waveguide into the interiorof the waveguide in a first longitudinal plane, said first probe beingadapted to receive a first signal polarised in said first longitudinalplane,

reflector means extending from the wall of the waveguide, said reflectormeans located downstream of said first probe and lying in said firstlongitudinal plane for reflecting signals polarised in said firstlongitudinal plane back to said first probe and allowing signalspolarised in a second plane orthogonal to said first longitudinal planeto pass along the waveguide,

a second probe located downstream of said reflector means and extendingfrom said wall of said waveguide into the interior of said waveguide andlying in said first longitudinal plane,

signal reflecting and rotating means, including a short circuit at theend of the waveguide, located downstream of said second probe forreceiving, rotating and reflecting a second signal polarised in saidsecond plane back along said waveguide such that said rotated andreflected signal is polarised in said first longitudinal plane and isreceived by said second probe,

said first and second probes having respective first and second outputslocated on the outside of the waveguide, the first and second outputslying in substantially said first longitudinal plane characterised inthat said reflecting and rotating means has a leading edge oriented atan angle of 45° to said first longitudinal plane and configured toprovide at least two reflecting edge portions thereon, said edgeportions being spaced at different distances from said short circuit atthe end of said waveguide whereby a portion of said second signal isreflected from each of said reflecting edge portions for recombinationwith the portion of said second signal reflected from said short circuitto provide a signal polarised in said first longitudinal plane fordetection by said second probe.

Preferably, said at least two reflecting edge portions are provided byspaced steps of equal width which are generally orthogonal to thewaveguide axis of the waveguide. Alternatively, the reflected edgeportions are provided by three spaced reflecting edges of equal length.The edges may be of different lengths.

Conveniently, the reflecting edges are orthogonal to the waveguide axisand are spaced from the short circuit by a predetermined distance forminimising signal loss across the required bandwidth.

In yet a further modification the reflecting edge may be provided by anedge which is not orthogonal to the waveguide axis, for example anoblique edge or a curved edge.

According to another aspect of the present invention there is provided amethod of receiving at least two orthogonally polarised signals in thefrequency range 10.7-12.75 GHz. in a single waveguide and providing atleast two outputs in a common longitudinal plane, said method comprisingthe steps of,

providing a first probe in a first longitudinal plane in said waveguideto receive a first signal polarised in said first longitudinal plane,

providing a reflector means in said waveguide parallel to and downstreamfrom said first probe for reflecting said first signal and for allowinga second signal polarised in a second plane orthogonal to said firstlongitudinal plane to pass,

providing a second probe in said waveguide parallel to and downstream ofsaid reflector means, said second probe being substantially orthogonalto said second plane to allow signals polarised in said second plane topass without being received by said second probe,

providing a rotating and reflector means at the end of the waveguidedownstream of said second probe with a waveguide short circuitdownstream of the reflector means, for receiving said second signal andfor reflecting said second signal back along said waveguide towards saidsecond probe, said rotating and reflecting means being oriented at anangle of 45° to said first longitudinal plane, said second signal alsobeing rotated to be polarised in said first longitudinal plane and to bereceived by said second probe,

and taking outputs from the first and second probes on the outside ofwaveguide, the outputs being disposed substantially in said firstlongitudinal plane, characterised in that said method includes the stepsof reflecting a portion of said second signal from each of saidreflecting edge portions and a portion of said second signal from saidshort circuit at the end of said waveguide, the reflected signalportions being phase shifted so that they recombine to provide aresultant signal in said first longitudinal plane for detection by saidsecond probe.

DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will become apparent from thefollowing description when taken in combination with the drawings inwhich:

FIG. 1 is a partly broken-away view of a low-noise block receiver with awaveguide probe including a reflecting twist plate in accordance with apreferred embodiment of the present invention;

FIG. 2 is a cross-sectional view of the waveguide taken on section 2--2of FIG. 1;

FIGS. 3a, b and c are graphs comparing the responses of a twist platewith a single reflecting surface and with two reflecting surfaces whereFIG. 3a, is a graph of a transmission loss versus frequency, FIG. 3b isa graph of phase shift of the signal hitting the leading edge of thetwist plate compared to the short circuit versus frequency, and FIG. 3cis a graph of signal return loss in dB. versus frequency, and

FIGS. 4a to h are side views of reflecting twist plates with multiplereflecting surfaces in accordance with alternative embodiments of theinvention.

DETAILED DESCRIPTION

Reference is first made to FIG. 1 of the drawings in which a low-noiseblock receiver, generally indicated by reference numeral 10, is adaptedto be mounted to a satellite receiving dish in a way which is well knownin the art. As is also known, the low-noise block receiver 10 isarranged to receive high frequency radiation signals from the satellitedish and to process these signals to provide an output which is fed to acable 12 which is, in turn, connected to a satellite receiver decoderunit (not shown in the interests of clarity).

The block receiver 10 includes a waveguide 14 which is shown partlybroken away to depict the interior components. The waveguide iscylindrical and is made of metal. The waveguide has a front aperture 16for facing a satellite dish for receiving electromagnetic radiation froma feed horn 18, shown in broken outline, which is mounted on the frontof the waveguide. The waveguide is substantially the same as thatdisclosed in applicant's co-pending Published International ApplicationWO92/22938. Thus, disposed within the waveguide in the same longitudinalplane is a first probe 20, a reflective post 22 and a second probe 24.In this embodiment, it will also be appreciated that the reflective post22 does not extend the entire diameter of the interior of the waveguidefor reasons disclosed in the aforementioned WO92/22938 specification.The outputs of the probes 20 and 24 pass through the waveguide wall 26along the same longitudinal plane generally indicated by referencenumeral 28. The probes 20,24 are of the same length so that the outputslie along the same longitudinal axis within the longitudinal plane 28.The distance between the probe 20 and reflective post 22 and probe 24and reflective post 22 is nominally λ/4 where λ is the wavelength of thesignals in the waveguide.

At the downstream end of the waveguide which is the furthest end fromthe front aperture, there is disposed within the waveguide a reflectingand rotating or twist plate 30. As best seen in FIG. 2 the reflectingand rotating plate is oriented at an angle of 45° to the probes 20,24and post 22. The furthest end of the plate terminates in wall 32 whichacts as a short circuit which will be later explained in detail.

It will be seen that the reflecting plate is thin and has a leading edgeformed of two step edges 34a,b of equal length and about the samethickness. The step edges 34a,b are orthogonal to the waveguide axis.Step 34a is further from the short circuit 32 than step 34b. With thisarrangement it will be appreciated that there are two reflective edgesat the leading end of the reflecting plate spaced by different amountsfrom wall 32.

In operation, signals from a satellite dish enter the waveguide 14 viathe horn 18 and aperture 16 and in accordance with known principles aretransmitted along the waveguide 14. The signals which are broadcast bythe satellite include two sets of signals which are orthogonallypolarised in the same frequency band and these are represented byvectors V1 and V2 which are signals polarised in the vertical andhorizontal planes respectively. As the signals travel along thewaveguide the vertically polarised signal V1 is received by the firstprobe 20 which, as it is spaced by λ/4 from the reflecting post 22,ensures the maximum field at the probe and hence optimum coupling to theprobe. The probe 20 has no effect on the horizontally polarised signalV2 which continues to pass along the waveguide.

As the reflecting post is vertically oriented the signal V2 is notreflected by the post and continues to pass along the waveguide 14 andalso passes the second probe 24 for the same reason. As the horizontallypolarised signal V2 passes along the waveguide it encounters step edge34a,b, of the thin metal twist plate 30 which is about 1-1.5 mm. thick.When the horizontally polarised signal V2 encounters the plate 30, onecomponent V2_(p) of the signal parallel to the plate encounters edges34a,b; a first portion of the component is reflected by edge 34a and asecond portion is reflected by edge 34b. The orthogonal component toV_(2P), V₂₀, is reflected by the short circuit 32 at the rear of theplate and is rotated by 180° shown as vector V20_(R) in broken outlinein FIG. 2. The distance of step 34a from short circuit 32 corresponds toa quarter of a wavelength (λ₁ /4) of a first frequency (f₁) near thelower end of the Astra frequency band and the distance of the step 34bfrom short circuit 32b corresponds to wavelength (λ₂ /4) of frequency f₂at the upper end of the frequency band. The signals reflected from edges34a,34b are out of phase and are represented by phase shifted vectorV2_(PRa), V2_(PRb). The reflected signal (V_(2OR)) is recombined withthe short circuit reflected signals to create a recombined vectorV2_(RCOMB), shown in broken outline, in the plane of probes 20,24. Thereflected and recombined signal indicated by vector V_(2RCOMB) thentravels towards probe 24 in the longitudinal plane which is received byprobe 24 and conducted to the probe output. Probe 24 is spaced from post22 by a quarter of a wavelength which ensures maximum field at the probeand hence optimum coupling.

With this arrangement it will be understood that the total signalreceived at probe 24 consists of a combination of reflected and rotatedsignals and because the signal components from edges 34a,34b are notin-phase, the amplitudes on recombination may be less, in some cases,than the amplitude for a single straight reflecting edge as in the priorart. The reduction in signal amplitude is not significant. However, theisolation provided by this waveguide with the stepped reflecting twistplate is not substantially different to that disclosed in theapplicant's aforementioned publication WO92/22938.

With this arrangement it will be appreciated that for differentfrequencies of transmitted signal the spacing between the various stepsand short circuit corresponds more closely to particular wavelengths.Thus the waveguide is tunable by selecting the distance of step 34a at adistance λ/4 from the short circuit 32 where λ corresponds to afrequency at the lower end of the frequency range, for example 11.0 GHz.and step 34b is set at a distance to correspond to wavelengths at ahigher frequency, for example 12.2 GHz. Such a bandwidth in a singlewaveguide was not possible with the aforementioned prior art waveguideand reflecting twist plate because of the single distance of the leadingedge from the short circuit corresponding to a quarter wavelength at asingle frequency. Thus, the stepped arrangement disclosed in FIGS. 1 and2 allows the low-noise block to be used to receive a wider range offrequencies; the bandwidth of the detector is substantially increased.There is, however, some loss in signal amplitude but in practice thishas been found to be quite acceptable for this application.

Reference is now made to FIGS. 3a,b,c which compare the response of awaveguide with a single edge reflector as in the prior art with awaveguide having the two step reflector plate shown in FIGS. 1 and 2.The two step plate is 18.5 mm wide (the width of the waveguide 14) andthe first step 34a is 15.1 mm from the short circuit 32 and the secondstep 34b is 7 mm from the short circuit. The length of each step is 9.25mm and the plate 30 is approximately 1 mm thick.

FIG. 3a shows transmission loss (dB.) with frequency with the graphsshowing the limits of the new Astra band 10.7 and 12.75 GHz.respectively. It will be seen that the response of the single reflectorfalls off as it approaches the lower and, more particularly, the upperband limits. The loss of about 2 dB. at the high end is unacceptable. Incontrast, it will be seen that the loss with the two step plate is muchless than 1 dB. and there is also minimum transmission loss at thecentre frequency.

Similarly, FIG. 3b shows that the phase shift deviation from 180° forthe two step plate above the mid-range is less than with the single stepplate which means that more signal is recombined with the correct phaseshift across the frequency range.

FIG. 3c is a graph of signal return loss (dB.) versus frequency whichshows that the minimal signal loss occurs at the single frequency with asingle plate, that is, the frequency corresponding to the λ/4 distanceof the edge from the short circuit. In contrast the response from thetwo step plate shows that minimal signals occur at a different frequencyand that there is a broader band of frequency for minimal return losswhich at the upper end of the frequency range shows at least a 5 dB.improvement over the single plate reflector.

Reference is now made to FIGS. 4a to h of the drawings which depict sideviews of alternative designs of reflector twist plates. It will be seenthat a twist plate with three steps may be used as shown in FIG. 5a, orfour steps as shown in FIG. 4b. In addition, it will be appreciated thatvariable reflecting edges may be created by machining out the twistplate to form an E-type profile as shown in FIG. 4c. This E-type profilemay be modified by a deeper recess as shown in FIG. 4d. It will also beunderstood that reflecting surfaces need not be orthogonal to thewaveguide axis. The leading edge may be provide by an oblique edge asshown in FIG. 4e or a curved edge as shown in FIG. 4f. The reflectingedges may be a combination of orthogonal or oblique edges or curves asshown in FIGS. 4g and 4h. In another embodiment the reflective post canalso extend across the entire waveguide; the waveguide operatingsatisfactorily with this structure.

It will be appreciated that the principal advantage of the presentinvention is that the reflecting plate allows the LNB to be used acrossa much greater bandwidth than the aforementioned prior art LNB.Consequently, a single LNB may be used to detect signals across all ofthe presently useable satellite bandwidths between 10.7 and 12.75 GHz. Afurther advantage of this arrangement is that it can use existingmanufacturing techniques and involves the selection of an appropriateplate for casting into the waveguide. The technique would be applicableto bandwidth improvement at other frequency ranges outside the Astrarange.

An improved dual polarization waveguide probe system has thus beendiscussed including a reflective twist plate 30 within a probe housing14 and which has at least at least two signal reflecting edges 34a and34b so that at least two separate signal reflections are created. Themultiple signal reflections enable the probe system to operate over awider frequency range with reduced deterioration in signal output. In apreferred arrangement, this can be achieved by making the reflectingtwist plate stepped and by providing two steps 34a and 34b spaced atdifferent distances from the waveguide short circuit 32. The leading,reflecting, edges of the steps 34a and 34b are orthogonal to thewaveguide axis.

That which is claimed is:
 1. A waveguide into which at least twoorthogonally polarised signals are received for transmission therealong,said waveguide comprising:a first probe extending from a wall of thewaveguide into the interior of the waveguide in a first longitudinalplane, said first probe being adapted to receive a first signalpolarised in said first longitudinal plane; reflector means extendingfrom the wall of the waveguide, said reflector means located downstreamof said first probe and lying in said first longitudinal plane forreflecting signals polarised in said first longitudinal plane back tosaid first probe and allowing signals polarised in a second planeorthogonal to said first longitudinal plane to pass along the waveguide;a second probe located downstream of said reflector means and extendingfrom said wall of said waveguide into the interior of said waveguide andlying in said first longitudinal plane; and signal reflecting androtating means, including a short circuit at the end of the waveguide,located downstream of said second probe for receiving, rotating andreflecting a second signal polarised in said second plane back alongsaid waveguide such that said rotated and reflected signal is polarisedin said first longitudinal plane and is received by said second probe;said first and second probes having respective first and second outputslocated on the outside of the waveguide, the first and second outputslying in substantially said first longitudinal plane wherein saidreflecting and rotating means has a leading edge oriented at an angle of45° to said first longitudinal plane and configured to provide at leasttwo reflecting edge portions thereon, said edge portions being spaced atdifferent distances from said short circuit at the end of said waveguidewhereby a portion of said second signal is reflected from each of saidreflecting edge portions for recombination with the portion of saidsecond signal reflected from said short circuit to provide a signalpolarised in said first longitudinal plane for detection by said secondprobe.
 2. A waveguide as claimed in claim 1 wherein said at least tworeflecting edge portions are provided by spaced steps of equal widthwhich are generally orthogonal to the waveguide axis of the waveguide.3. A waveguide as claimed in claim 1 wherein the reflecting edgeportions are provided by three spaced reflecting edges of equal length.4. A waveguide as claimed in claim 1 wherein the edge portions are ofdifferent lengths.
 5. A waveguide as claimed in claim 1 wherein thereflecting edge portions are orthogonal to the waveguide axis and arespaced from the short circuit by a predetermined distance for minimisingsignal loss across the required bandwidth.
 6. A waveguide as claimed inclaim 1 wherein at least one reflecting edge portion is provided by anedge which is not orthogonal to the waveguide axis.
 7. A method ofreceiving at least two orthogonally polarised signals in the frequencyrange 10.7 GHz to 12.75 GHz in a single waveguide and providing at leasttwo outputs in a common longitudinal plane, said method comprising thesteps of;providing a first probe in a first longitudinal plane in saidwaveguide to receive a first signal polarised in said first longitudinalplane; providing a reflector means in said waveguide parallel to anddownstream from said first probe for reflecting said first signal andfor allowing a second signal polarised in a second plane orthogonal tosaid first longitudinal plane to pass; providing a second probe in saidwaveguide parallel to and downstream of said reflector means, saidsecond probe being substantially orthogonal to said second plane toallow signals polarised in said second plane to pass without beingreceived by said second probe; providing a rotating and reflector meansat the end of the waveguide downstream of said second probe with awaveguide short circuit downstream of the reflector means, for receivingsaid second signal and for reflecting said second signal back along saidwaveguide towards said second probe, said rotating and reflecting meansbeing oriented at an angle of 45° to said first longitudinal plane, saidsecond signal also being rotated to be polarised in said firstlongitudinal plane and to be received by said second probe; and takingoutputs from the first and second probes on the outside of waveguide,the outputs being disposed substantially in said first longitudinalplane; and reflecting a portion of said second signal from said rotatingand reflector means and a portion of said second signal from said shortcircuit at the end of said waveguide, the reflected signal portionsbeing phase shifted so that they recombine to provide a resultant signalin said first longitudinal plane for detection by said second probe. 8.A waveguide comprising:a waveguide body defining a waveguide cavitytherein wherein said waveguide cavity has an aperture at a first endthereof, and wherein said waveguide cavity has a waveguide axistherethrough extending from said first end to a second end; a twistplate in said waveguide cavity at said second end of said waveguidecavity wherein said twist plate is parallel to said waveguide axis,wherein said twist plate includes a leading edge facing said aperture,and wherein said leading edge includes first and second portions withsaid second portion being more distant from said aperture than saidfirst portion; a first probe in said waveguide cavity between saidaperture and said leading edge of said twist plate for receiving a firstsignal having a first polarization entering said aperture; and a secondprobe in said waveguide cavity between said first probe and said leadingedge of said twist plate for receiving a second signal having a secondpolarization entering said aperture.
 9. A waveguide according to claim 8further comprising a short circuit at said second end of said waveguidecavity wherein said short circuit is adjacent said twist plate oppositesaid aperture.
 10. A waveguide according to claim 8 further comprising areflective post in said waveguide cavity between said first and secondprobes, wherein said first and second probes and said reflective postlie in a common longitudinal plane, and wherein said leading edge ofsaid twist plate lies in a second plane oriented at a 45 degree anglewith respect to said common longitudinal plane.
 11. A waveguideaccording to claim 8 further comprising a reflector between said firstand second probes wherein said reflector reflects electromagneticradiation having said first polarization.
 12. A waveguide according toclaim 11 wherein said reflector comprises a reflective post, whereinsaid first and second probes and said reflective post lie in a commonlongitudinal plane, wherein said first polarization is aligned in saidlongitudinal plane, and wherein said second polarization is parallel tosaid longitudinal plane.
 13. A waveguide according to claim 8 whereineach of said first and second portions of said leading edge of saidtwist plate are orthogonal with respect to said waveguide axis.
 14. Awaveguide according to claim 9 wherein said waveguide is adapted toreceive signals over a range of frequencies between a low frequency anda high frequency, wherein said first portion of said leading edge isspaced from said short circuit by a distance of one quarter of awavelength of said low frequency, and wherein said second portion isspaced from said short circuit by a distance of one quarter of awavelength of said high frequency.
 15. A waveguide according to claim 14wherein said low frequency is approximately 10.7 GHz and wherein saidhigh frequency is approximately 12.75 GHz.
 16. A waveguide according toclaim 8 wherein said leading edge of said twist plate further includes athird portion between said first and second portions wherein said thirdportion is not orthogonal with respect to said waveguide axis.
 17. Awaveguide according to claim 8 further comprising an electrical couplingbetween said first and second probes and an electrical cable outsidesaid waveguide cavity.
 18. A receiver comprising:a wave guide including,a waveguide body defining a waveguide cavity therein wherein saidwaveguide cavity has an aperture at a first end thereof, and whereinsaid waveguide cavity has a waveguide axis therethrough extending fromsaid first end to a second end, a twist plate in said waveguide cavityat said second end of said waveguide cavity wherein said twist plate isparallel to said waveguide axis, wherein said twist plate includes aleading edge facing said aperture, and wherein said leading edgeincludes first and second portions with said second portion being moredistant from said aperture than said first portion, a first probe insaid waveguide cavity between said aperture and said leading edge ofsaid twist plate for receiving a first signal having a firstpolarization entering said aperture, and a second probe in saidwaveguide cavity between said first probe and said leading edge of saidtwist plate for receiving a second signal having a second polarizationentering said aperture; and a decoder electrically coupled to said firstand second probes.
 19. A receiver according to claim 18 wherein saidwaveguide further includes a short circuit at said second end of saidwaveguide cavity wherein said short circuit is adjacent said twist plateopposite said aperture.
 20. A receiver according to claim 18 whereinsaid waveguide further includes a reflective post in said waveguidecavity between said first and second probes, wherein said first andsecond probes and said reflective post lie in a common longitudinalplane, and wherein said leading edge of said twist plate lies in asecond plane oriented at a 45 degree angle with respect to said commonlongitudinal plane.
 21. A receiver according to claim 18 wherein saidwaveguide further includes a reflector between said first and secondprobes and wherein said reflector reflects electromagnetic radiationhaving said first polarization.
 22. A receiver according to claim 21wherein said reflector comprises a reflective post, wherein said firstand second probes and said reflective post lie in a common longitudinalplane, wherein said first polarization is aligned in said longitudinalplane, and wherein said second polarization is parallel to saidlongitudinal plane.
 23. A receiver according to claim 18 wherein each ofsaid first and second portions of said leading edge of said twist plateare orthogonal with respect to said waveguide axis.
 24. A receiveraccording to claim 19 wherein said waveguide is adapted to receivesignals over a range of frequencies between a low frequency and a highfrequency, wherein said first portion of said leading edge is spacedfrom said short circuit by a distance of one quarter of a wavelength ofsaid low frequency, and wherein said second portion is spaced from saidshort circuit by a distance of one quarter of a wavelength of said highfrequency.
 25. A receiver according to claim 24 wherein said lowfrequency is approximately 10.7 GHz and wherein said high frequency isapproximately 12.75 GHz.
 26. A receiver according to claim 18 whereinsaid leading edge of said twist plate further includes a third portionbetween said first and second portions wherein said third portion is notorthogonal with respect to said waveguide axis.
 27. A receiver accordingto claim 18 further comprising a receiving dish oriented to reflectelectromagnetic radiation toward said aperture of said waveguide cavity.28. A receiver according to claim 27 wherein said receiving dish isadapted to reflect electromagnetic radiation transmitted by a satellite.